Portable electronic devices, such as video cameras, cellular phones, notebook computers, etc., are becoming more lightweight and demanding increasingly high performance. As a result, more research is being conducted into batteries for use as power supplies for such portable devices. In particular, rechargeable lithium secondary batteries are actively being researched because they have 3 times as much energy density per unit weight as conventional lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries, etc., and can be rapidly charged.
In lithium ion batteries, transition metal compounds such as LiNiO2, LiCoO2, LiMn2O4, LiFePO4, LiNixCo1−xO2 (x=1, 2), Ni1−x−yCoxMnyO2 (0≦x≦0.5, 0≦y≦0.5) and lithium oxides thereof can be used as cathode active materials. Lithium metals, lithium alloys, carbonaceous materials, graphite materials, etc. can be used as anode active materials.
Electrolytes can be classified as liquid electrolytes or solid electrolytes. Liquid electrolytes present risks of fire due to leakage of the electrolytic solution and battery breakage due to vaporization of the electrolytic solution. However, solid electrolytes do not leak and can be easily processed. A lot of research has been conducted into solid electrolytes, such as solid polymer electrolytes. Solid polymer electrolytes can be classified as complete solid electrolytes containing no organic electrolytic solution or gel-type electrolytes containing organic electrolytic solution.
Lithium batteries are generally driven at high operating voltages. Accordingly, conventional aqueous electrolytic solutions cannot be used because the aqueous solution vigorously reacts with the lithium contained in the anode. In place of aqueous electrolytic solutions, organic electrolytic solutions in which lithium salts are dissolved in organic solvents are used. Organic solvents having high ionic conductivity, high dielectric constants and low viscosity are used. Since it is difficult to find a single organic solvent having all of these characteristics, a mixed solvent is generally used. For example, the mixed solvent can include an organic solvent with a high dielectric constant and an organic solvent with a low dielectric constant. Alternatively, the mixed solvent can include an organic solvent with a high dielectric constant and an organic solvent with a low viscosity.
Lithium secondary batteries form passivation layers such as solid electrolyte interface (SEI) films on the negative electrode surfaces upon initial charging. The SEI films are formed by the reaction of carbon in the anode with the electrolytic solution. The SEI film enables stable charging and discharging of the battery without further decomposition of the electrolytic solution (J. Power Sources, 51(1994), 79-104). Also, the SEI film acts as an ion tunnel through which only lithium ions pass. The film solvates the lithium ions to prevent co-intercalation of the organic solvent, which moves with the lithium ions into the carbonaceous anode, thereby preventing breakdown of the anode structure.
However, upon initial charging, gas is generated inside the battery due to decomposition of the carbonate-based organic solvent during formation of the SEI film. This gas generation results in an increase in battery thickness (J. Power Sources, 72 (1998), 66-70). When the lithium battery is stored at high temperatures after charging, the passivation layer gradually breaks down due to electrochemical energy and thermal energy which increases over time. The breakdown of the passivation layer exposes the anode surface, causing the amount of gas generated to increase. The generation of gas also causes the internal pressure of the battery to increase, which causes deformation of the central portion of the side of the battery (such as swelling of a rectangular lithium polymer battery in a certain direction). The increased internal pressure of the battery results in decreased adherence between electrode plates, thereby reducing the performance and safety of the battery and making the mounting of a set of lithium secondary batteries difficult.
To address these concerns, surfactants have been proposed for use in the electrolytic solution. However, the surfactants used generally include alkyl groups as nonpolar groups for the hydrophobic parts, and ionic groups or the like as polar groups. The anode mainly comprises carbon-based material having repeating aromatic group structures. The electrolyte mainly includes a carbonate-based solvent. When a conventional surfactant is used at the interface between the anode and electrolyte good active surface properties are not observed due to the differences between the structure of the surfactant and the structure of the medium. Thus, conventional surfactants do not provide a good barrier between the negative electrode and the electrolyte and do not adequately assist the re-impregnation of a squeezed electrolyte. Therefore, a need exists for a surfactant which displays improved active surface properties in non-aqueous environments.